The present invention relates to a circuit for monitoring the state of battery in a cardiac pacemaker, particularly a circuit of the type including a voltage detector which emits a signal as soon as a value characterizing the battery state passes a threshold.
In artificial cardiac pacemakers, such circuits serve to indicate the approaching end of the operating life (EOL) of the battery so that the necessary measures can be taken in time. A widespread way of achieving this is by constructing the generator of the cardiac pacemaker, which generates stimulation pulses, to have a frequency characteristic which is such that its pulse rate has a fixed value during normal operation and decreases with decreasing battery voltage. Once the pulse rate drops below a given value, replacement of the battery is necessary.
When CMOS circuits or quartz controlled oscillators are used to determine the stimulation pulse rate, their frequency does not change noticeably with a change in the battery voltage. To nevertheless obtain an indication of the battery state it is known to provide additional voltage dependent trigger circuits in the circuitry of cardiac pacemakers with the additional circuits emitting a signal when the battery voltage drops below a given value. These known circuits evaluate only the terminal voltage of the battery. However, due to the internal resistance of the battery, the terminal voltage existing when the battery load is drawing current depends on the current consumption necessitated by the particular operating state of the pacemaker. Since, in particular, programmable pacemakers have widely varying operating behaviors, the corresponding current consumption fluctuates considerably. Even with the conventional demand pacemakers, current consumption changes at a ratio of 5:1 during transition from inhibited to stimulating operation.
In a borderline region toward the end of the operating life of the battery it may then happen that in one mode of operation the approaching end of the battery life is already being indicated while this is not yet the case in the other mode of operation. This presents the danger that if the pacemaker continues to operate for a longer period of time in a relatively inactive mode, nothing indicates the approaching exhaustion of the battery so that if, as a result of cardiac behavior, a longer phase of operation with greater pacemaker activity is required, the end of the operating life of the battery, although an appropriate warning signal is emitted, may be reached dangerously quickly since during the preceding mode of operation the battery charge had already been depleted to too great an extent without this having been noticed.
If the signal emitted by such a circuit to monitor the battery state is used to reduce the basic rate of the oscillator, which is generally independent of the operating voltage, by internal switching, the average current consumption of the pacemaker is reduced and the battery terminal voltage, which has been smoothed by filter members increases again. Thus the control circuit will no longer emit a signal and the basic rate of the pacemaker will be stepped up again. This feedback effect therefore produces a continuous change in frequency which is extremely annoying for the user of the pacemaker.